[0001] The invention relates to a process for producing a low-nitrogen synthesis
gas from a natural gas containing nitrogen and carbon dioxide, from which water
10 and carbon dioxide are removed in a first temperature swing adsorption plant and
subsequently nitrogen is removed in a cryogenic gas fractionator, to give a lownitrogen, water-free and carbon dioxide-free natural gas, which is next supplied to
a thermochemical conversion, so as to recover a crude syngas comprising
hydrogen, carbon monoxide, water and carbon dioxide, from which the low15 nitrogen synthesis gas is obtained at least by the removal of water and carbon
dioxide in a second temperature swing adsorption plant.
[0002] The invention also relates to a device for carrying out the method
according to the invention.
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[0003] In the terminology used herein, a gas mixture is considered to be free of a
component if the proportion of this component in the gas mixture does not exceed
0.1 ppmv. In contrast, a gas mixture is referred to as containing a component if the
proportion of this component is greater than 0.5 mol%. A gas mixture is
25 considered to be low in a component if the proportion of this component is
between 0.5 mol% and 1 ppmv.
[0004] Synthesis gases are mixtures of gases containing at least hydrogen and
carbon monoxide and can be used to synthesize various products. They are
30 predominantly produced from natural gas, which is thermochemically converted
by means of autothermal reforming (ATR), partial oxidation (POX), steam
reforming (SMR) or a combination of two or more of these processes that have
been known from the prior art for many years. The carbon dioxide-containing
natural gas, which usually has a nitrogen content of between 1 to 10 mol%, is
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prepared, for example, by means of mercury removal, desulfurization, heating and
compression to form a natural gas feedstock, which is subsequently converted,
with the addition of water and/or carbon dioxide, to a crude syngas that, in
addition to carbon monoxide and hydrogen, also comprises larger amounts of
5 carbon dioxide and water and other substances undesirable in the synthesis gas. To
obtain a low-nitrogen synthesis gas, nitrogen can be removed either downstream
of the thermochemical conversion from the crude syngas produced or already
upstream from the natural gas used.
10 [0005] If nitrogen removal takes place downstream of the thermochemical
conversion, the desulfurized and mercury-free natural gas is converted with its
entire nitrogen load into a nitrogen-containing crude syngas from which the
majority of the carbon dioxide is first removed, for example, in an amine scrubber,
before water and the remaining carbon dioxide residue are removed in a
15 temperature swing adsorption plant (referred to hereinafter as TSAP for short).
The gas mixture thus obtained, consisting largely of hydrogen, carbon monoxide
and nitrogen, is then separated with the aid of a cryogenic gas fractionator into
crude hydrogen, carbon monoxide and a nitrogen fraction containing combustible
substances. After its use as regenerating gas in the TSAP, the crude hydrogen is
20 purified in a pressure swing adsorption plant (PSAP for short) to produce pure
hydrogen, at least a part of which is mixed with carbon monoxide from the
cryogenic gas fractionator to produce the low-nitrogen synthesis gas.
[0006] In the cryogenic gas fractionator, carbon monoxide and nitrogen are
25 separated in a column by rectification, which can only be achieved with a high
reflux ratio and/or many separation stages in the column due to the very similar
boiling temperatures of the two substances. A further compressor is necessary in
order to recirculate the hydrogen-containing residual gas produced during the
purification of the crude hydrogen in the PSAP and to use it in the
30 thermochemical conversion or to burn it as a fuel in order to achieve a sufficiently
high yield. The resulting high apparatus and energy requirements negatively affect
the economic viability of this process variant.
[0007] Because the boiling temperatures of nitrogen and methane differ from one
35 another significantly more strongly than those of nitrogen and carbon monoxide, it
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is relatively easy to remove nitrogen from nitrogen-containing natural gas by
rectification in a cryogenic gas fractionator. In particular, the separating column
used can be operated in an energetically favorable manner without reflux. An
alternative process variant therefore provides for the nitrogen not to be separated
5 from the crude syngas, but rather from the natural gas in a cryogenic process, and
to produce, by thermochemical conversion, a likewise low-nitrogen crude syngas
from the low-nitrogen natural gas feedstock thus obtained. When processing the
crude syngas to produce the low-nitrogen synthesis gas, this process variant still
requires a device for removal of carbon dioxide and a TSAP for water and carbon
10 dioxide removal, but a complex cryogenic synthesis gas fractionator, a PSAP for
purifying crude hydrogen and a recycle compressor for the hydrogen-containing
PSAP residual gas can be dispensed with. However, it is disadvantageous that at
least one additional TSAP and possibly a carbon dioxide removal device must be
arranged upstream of the cryogenic natural gas fractionator, which TSAP and
15 carbon dioxide removal device prevent water and carbon dioxide from entering
the cryogenic natural gas fractionator together with the natural gas, where they
would freeze and cause blockages. In particular when carbon dioxide is removed
by means of a method based on an aqueous washing agent such as amine
scrubbing, water saturation occurs such that a TSAP for water removal is required
20 even if the natural gas used is anhydrous.